This proposal is to study basic neural mechanisms of the brain and spinal cord that are used in the control of motor output. The experiments will be carried out with macaque monkeys that are trained to execute a general tracking task; the monkeys are required to compensate for changes in force and to track changes in target position. A major portion of the proposed experiments is directed toward a microelectrode study of the role of magnocellular and parvocellular neurons of the red nucleus in the control of muscle length and tension. Specifically, relations between discharge rate, limb position and force will be evaluated; stimulus-response properties will be studied particularly with proprioceptive stimuli; and an attempt will be made to evaluate the role of this nucleus in controlling both motoservo actions and reaction-time movements.